Originally Posted by GUWonder

All of the above and more.

There are lots of permutations to account for differences in the names between the US and non-US passports of US dual-citizens; and the differences go along these lines (in no particular order): name order; transliteration to factor in for alphabet differences between languages or pronunciation differences; distribution of names across each name field use; national name prioritization factors where first given name isn’t the person’s primary name; name amendment listings in passport amendment section; legal standards for names (and/or name changes) being more restrictive or prescriptive in one jurisdiction than the other (and thus names, name order, name spelling, name field entry use come with the name differences across the dual-citizens passports from each country); and having more names in one jurisdiction than in another. These differences mean there is often a more manual check-in aspect involved to travel for dual-citizens than for non-dual citizens when using round trip tickets. On one-way tickets, some just do for names what works easiest for self check-in online or at airline kiosks without as high a chance of needing to talk as much to an airline check-in rep.